1. Field of the Invention
The present invention is of a sterility indicator, also known as a biological indicator. The invention has application in hospitals, laboratories, etc., where sterilized equipment is necessary.
2. Discussion of the Prior Art
Sterility indicators have been used for many years in hospitals and other environments where utensils and equipment free from spores and microorganisms are required to prevent infection, the spreading of viruses, etc. Even disposable surgical utensils must be sterilized before they are packaged.
Of course, it is impossible to tell whether a utensil is sterile simply by looking at it. What is less obvious is the fact that all but the most prolonged exposure of utensils to a sterility cycle gives an acceptable probability of adequate sterilization. For "quicker" sterilization cycles, the percentage of unsterile results becomes intolerable. Subjecting all utensils requiring sterilization to prolonged cycles is inefficient, uneconomical, and time consuming. Sterility indicators were developed to allow the widespread use of "quick" sterilization cycles since their use indicates whether a sterilization cycle has been successful or not.
The prior art has developed a number of compact, disposable sterility indicators. These indicators usually come in a sealed unit which is subjected along with the utensils to a sterility cycle. The sterility cycle may be a steam cycle or an ethylene oxide (EO) cycle, all known to the art. After the cycle, the indicator is treated, resulting in an indication of whether the cycle was successful
In a gravity steam sterilization cycle, steam diffuses throughout the sealed compartment in which the items to be sterilized are placed. In a vacuum steam cycle, air is evacuated from the compartment before the steam is pumped into the compartment; the evacuation and pumping may be repeated several times prior the sterilization cycle. The vacuum eliminates the local pockets of air in the compartment which resist steam penetration in the gravity methods
Wet steam more effectively kills microorganisms than dry steam, so the relative humidity (RH) of the steam cycles is usually 100%. Other variables effecting the efficacy of a steam cycle are time, temperature and whether vacuum techniques are used. For a gravity cycle at 121.degree. C., typically the instruments are exposed for a minimum of 15 minutes. For the so called "flash" gravity cycle, instruments are typically exposed for 3 minutes at a minimum of 132.degree. C.
In the EO cycles, air is typically removed from the chamber prior to introduction of the gas or gas mixture. EO cycles are dependent on time, concentration, RH, and temperature. For a concentration of 1200 mg/liter EO at 25.degree. C., 1 atm and 60% RH, the instruments are typically exposed for several hours depending on the load, content and size.
A typical biological sterility indicator has a hollow outer compartment with viable microorganisms disposed therein. Disposed within a frangible inner compartment and isolated from the spores is a nutrient medium which spurs growth of the spores when the device is actuated and some type of means to indicate the growth of microorganisms. The outer compartment generally has some mechanism whereby a gas may pass into the outer compartment. Therefore, the gas or steam of a sterilization cycle may pass into the interior of the outer compartment to kill the microorganisms during the sterilization cycle.
For sterility indicators which will be subjected to a steam cycle, the spores are usually Bacillus stearothermophilus since they have been demonstrated to be most resistant to steam sterilization. They usually reside the spores are usually Bacillus subtilis, and reside on strips with 10.sup.6 or more spores
In principle and in operation the sterility indicator is subjected to the same sterilization cycle as the utensils. The gas or steam of the cycle passes into the interior of the outer compartment, thus theoretically exposing the spores to the same sterilizing medium as any on the utensils. After the sterilization cycle, the nutrient medium is brought into contact with the spores by actuation of a mechanism which shatters the frangible inner compartment, thereby releasing the nutrient medium. The sterility indicator is then subjected to an incubation process. Any spores which survived the sterilization cycle will grow and be revealed by the indicating means. Naturally such an indication means an unacceptable sterilization cycle, while lack of an indication means the opposite.
The nutrient medium for both types of spores may be Tryptic Soy broth The detection mediums may be Bromthymol blue or purple, which change color in acidic conditions, i.e. upon growth of spores.
Incubation requires high humidity to retard drying of the spores, which retards the growth of those that survive the cycle. Typically the indicator is incubated for 24 hours and checked for a color change indicating growth. If there is no sign of growth, the indicator is incubated another 24 hours and again checked for growth.
U.S. Pat. No. 3,661,717 to Nelson exemplifies a particular embodiment of a sterility indicator. The outer compartment is a hollow cylinder with one closed end and one open end. The outer compartment is made of translucent, deformable plastic Disposed within the outer compartment is a closed cylindrical inner compartment made of frangible glass and containing the nutrient medium. Also disposed in the outer compartment is a strip with a predetermined number of viable microorganisms, and a material which indicates the growth of microorganisms through a color change. The open end of the outer compartment is closed with a liquid impervious, gas pervious sheet. Thus, the gas or steam of a sterilization cycle permeates through the sheet to the interior of the outer compartment, exposing the microorganisms on the strip to the same sterilization cycle as any microorganisms on the utensils. When the cycle is complete, any gas inside the outer compartment diffuses or is evacuated out through the sheet.
Once the cycle is complete, the frangible glass of the inner compartment is fractured by squeezing the walls of the outer compartment, thus exposing the strip to the nutrient medium. The liquid nutrient medium remains inside the outer compartment due to the sheet being liquid impervious. The unit is then incubated, and any color change in the detecting material caused by the growth of microorganisms is seen through the translucent walls of the outer compartment.
One disadvantage of this prior art device is the method of fracturing the inner compartment. To be deformable and translucent, the plastic walls of the outer compartment have to be relatively thin and, when squeezed, are susceptible to piercing by the fracturing glass of the inner compartment. This at times leads to injury to the operator as well as contamination of a sterile environment.
Another disadvantage of this prior art system is its propensity to give false negative readings. The combination of the sheet barrier and the deformable plastic walls of the outer compartment result in the strip having a longer exposure time to the gas of a sterilization cycle. Specifically, the sheet acts to some degree as a barrier to trap gas inside the outer compartment when the cycle is complete. Furthermore the plastic walls of the outer compartment absorb gas during the cycle, and release gas to the interior of the compartment during the incubation cycle. The longer exposure of the strip to the gas may result in the spores on the strip being killed while those on the utensils of interest are not. Therefore, the prior art device would indicate a successful sterilization cycle when in fact it was not.
A number of embodiments too numerous to mention attempt to solve the above problems inherent in this prior art device. A sampling of the devices can be found in U.S. Pat. Nos. 4,741,437, 4,416,984, 4,732,850, 4,461,837, 4,304,869, and 4,580,682. Many of these embodiments substitute safe but complex configurations for the relatively simple, and thus economical, inner and outer compartments of the prior art device. Some embodiments use a capsule shaped inner compartment similar to the prior art device, combined with a cylindrical outer compartment. The outer compartment in some way collapses along the cylindrical axis, thus crushing the inner compartment. Such a configuration is also inherently flawed since it attempts to break the inner compartment by applying pressure primarily along its cylindrical axis, the axis most resistant to fracture
The later embodiments also attempt to correct the false positive problem By making devices with outer compartments that collapse along the cylindrical axis to open the inner compartment, the outer compartment can be made of rigid material. This material does not absorb gas to the degree that the deformable plastics of the prior art devices do. However, these methods are disadvantageous because of their complexity and resulting cost, as detailed above.
Some of the later embodiments also attempt to correct the false positive problem by replacing the gas transmissive, liquid impervious sheet with another mechanism for exposing the spores to the sterility cycle. One mechanism is a "tortuous path." The tortuous path is broadly defined as an unobstructed passage between the exterior and interior of the outer compartment having a minimum of two 90.degree. bends in either path. The passage is "unobstructed" only in the sense that an object need not be passed through to move between the outside and inside of the outer compartment. The path is "tortuous" because it may twist around objects and physical projections in moving between the interior and exterior of the outer compartment. By substituting a tortuous path for a gas permeable sheet, the level of gas or steam inside the outer compartment is more reflective of the level outside the outer compartment at all phases of the cycle.
One problem with the prior art methods using the tortuous path is that many of those tortuous paths are not liquid impermeable. Thus when the inner compartment is accidently opened during shipping, handling or a sterility cycle, nutrient medium can leak to the outside of the sterility indicator. Considering that the nutrient medium is to spur the growth of microorganisms, such an occurrence in a hospital or clean environment is undesirable and detrimental to the future performance of the indicators.
Another problem with the prior art methods using the tortuous path was the propensity to give false positive results in the gravity steam or EO cycles. Because the prior art devices used only one tortuous path, the sterility medium was prevented from diffusing inside due to air trapped therein. As a result the spores on the strip were not exposed and killed, and the indicator after incubation would indicate growth for what may have been a successful cycle.